1. Field of the Invention
The present invention relates to an optical pickup system used in an information recording and/or reproducing apparatus, and more particularly to an optical pickup system for accessing different types of optical recording media and an information recording and/or reproducing apparatus employing the same.
2. Prior Art
In recent years, in order to satisfy ongoing requirements for recording and/or reproducing large quantities of data on recording media, many manufacturers have sought to increase the recording density of recording media. The recording density of a recording medium is determined by the size of a light spot illuminating the medium. Generally, the size of the light spot is proportional to the wavelength of the light, and inversely proportional to the numerical aperture (NA) of an objective lens that focuses the light. Therefore, reducing the wavelength or increasing the NA can increase the recording density of the recording medium.
An industry-wide standard relating to a next generation optical disk such as a high definition-digital versatile disk (HD-DVD) has been proposed to satisfy the demand for increased recording density of recording media. The HD-DVD standard employs a laser diode generating a blue laser with a wavelength of 405 nm, an objective lens having an NA of 0.85, and a light transmission protective layer of the optical disk having a thickness of 0.1 mm.
It is important to be able to employ a conventional compact disk (CD) and a conventional digital versatile disk (DVD) in an HD-DVD apparatus, because CDs and DVDs are still very popular whereas HD-DVDs are still relatively nascent. However, various optical conditions for recording/reproducing on/from CDs, DVDs, and HD-DVDs are different from each other, as shown in table 1.
TABLE 1CDDVDHD-DVDwavelength780nm650nm405nmnumerical aperture (NA)0.450.60.85recording capacity0.65GB4.7GBmore than20GBthickness of1.2mm0.6mm0.1mmprotective layer
As can be seen, different optical disks need different objective lenses with different NAs. Therefore in a single conventional HD-DVD apparatus, there are usually three different objective lenses respectively adapted to CDs, DVDs and HD-DVDs. However, this makes the volume of the HD-DVD apparatus unduly large. To avoid this shortcoming, another conventional HD-DVD apparatus with only one objective lens and a wavelength selector has been developed. The wavelength selector changes an effective diameter of the objective lens by means of limiting the luminous flux propagating to the objective lens. With the help of the wavelength selector, the objective lens in the HD-DVD apparatus is suitable for reading and/or reproducing not only with respect to HD-DVDs, but also with respect to DVDs and CDs.
An information recording and/or reproducing apparatus employing only one objective lens for accessing three different optical recording media is disclosed in US patent application publication no. 2003/0185136A1. This publication discloses an information recording and/or reproducing apparatus including three laser diodes, three photodiodes, three beam splitters, three condensing lenses, a wavelength-selecting unit, and an objective lens. The three diodes emit three laser beams with different wavelengths, e.g., 405 nm, 650 nm and 780 nm, to be used in recording and reproducing operations for HD-DVDs, DVDs and CDs respectively. The three photodiodes receive the three laser beams reflected from the three different optical disks respectively. The three condensing lenses respectively condense the three laser beams. The wavelength-selecting unit changes the luminous flux of laser beams propagating to the objective lens. The objective lens focuses the three laser beams on the three different optical disks. In this apparatus, the wavelength-selecting unit and the objective lens are in a common optical path for the three laser beams to propagate along. The size of the information recording and/or reproducing apparatus is reduced to a certain extent, because two objective lenses adapted to DVDs and CDs are not required and are omitted. The wavelength-selecting unit changes the effective diameter of the objective lens by means of limiting the luminous flux propagating to the objective lens. Therefore the objective lens can be used in recording and/or reproducing operations for the three different optical disks, with optical aberration being reduced to a certain extent.
However, the structure and the size of the information recording and/or reproducing apparatus are relatively complex and large, because numerous optical components are still required.